Electrolyte Additives

ABSTRACT

Described herein are additives for use in electrolytes that provide a number of desirable characteristics when implemented within batteries, such as high capacity retention during battery cycling at high temperatures. In some embodiments, a high temperature electrolyte includes a base electrolyte and one or more polymer additives, which impart these desirable performance characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Non-Provisional applicationSer. No. 15/441,892 filed Feb. 24, 2017 entitled “ElectrolyteAdditives”, the disclosure of which is hereby incorporated by referencein its entirety.

BACKGROUND OF THE INVENTION

The present invention is in the field of battery technology and, moreparticularly, in the area of additive compounds for use with high-energyelectrodes in electrochemical cells.

A liquid electrolyte serves to transport ions between electrodes in abattery. Organic carbonate-based electrolytes are most commonly used inlithium-ion (“Li-ion”) batteries and, more recently, efforts have beenmade to develop new classes of electrolytes based on sulfones, silanes,and nitriles. Unfortunately, these conventional electrolytes typicallyoften cannot be operated at high voltages and/or at high temperatures.At high voltages, conventional electrolytes can decompose, for example,by catalytic oxidation in the presence of cathode materials, to produceundesirable products that affect both the performance and safety of abattery. Conventional electrolytes may also be degraded by reduction bythe anodes when the cells are charged.

As described in more detail below, solvents, salts, or additives havebeen incorporated into the electrolyte to decompose on the electrode toform a protective film called a solid electrolyte interphase (SEI).Depending on the exact chemical system, this film can be composed oforganic or inorganic lithium salts, organic molecules, oligomers, orpolymers. Often, several components of the electrolyte are involved inthe formation of the SEI (e.g., lithium salt, solvent, and additives).As a result, depending on the rate of decomposition of the differentcomponents, the SEI can be more or less homogenous.

In past research, organic compounds containing polymerizable functionalgroups such as alkenes, furan, thiophene, and pyrrole had been reportedto form an SEI on the cathode of lithium ion batteries. See, e.g., Y.-S.Lee et al., Journal of Power Sources 196 (2011) 6997-7001. Theseadditives likely undergo polymerization during cell charging to formpassivation films on the electrodes. SEIs are known to contain highmolecular weight species. However, in situ polymerization during theinitial charge often cannot be controlled in a precise enough manner toprevent non-uniform SEIs comprised of polymer or oligomer mixtures witheither heterogeneous molecular weight, heterogeneous composition, oreven undesired adducts. The non-uniformity of the SEI often results inpoor mechanical and electrochemical stability, which is believed to be amain cause of cycle life degradation in lithium ion batteries. Thus, theimprovement in cell performance using these materials is limited.

Further, certain organic polymers have also been used as solidelectrolytes for lithium ion batteries due to the generally lowvolatility and safety of polymeric molecules as compared to smallerorganic molecules, such as organic carbonates. However, practicalapplication of such systems has been limited due to poor ionicconductivity.

For high-energy cathode materials, electrolyte stability remains achallenge. Recently, the need for better performance and higher capacitylithium ion secondary batteries used for power sources is dramaticallyincreasing. Lithium transition metal oxides such as LiCoO₂ (“LCO”) andLiNi_(0.33)Mn_(0.33)Co_(0.33)O₂ (“NMC”) are state-of-the-art high-energycathode materials used in commercial batteries. Yet only about 50% ofthe theoretical capacity of LCO or NMC cathodes can be used with stablecycle life. To obtain the higher capacity, batteries containing thesehigh-energy materials need to be operated at higher voltages, such asvoltages above about 4.2V. However, above about 4.3V, conventionalelectrolytes degrade and this leads to a significant deterioration ofthe cycle life. Further, the decomposition of the electrolyte at highervoltages can generate gas (such as CO₂, O₂, ethylene, H₂) and acidicproducts, both of which can damage a battery. These effects are furtherenhanced in “high nickel” NMC compositions such asLiNi_(0.6)Mn_(0.2)Co_(0.2)O₂ or LiNi_(0.8)Mn_(0.1)Co_(0.1)O₂ or otherswhich can provide higher capacities due to the electrochemistry of thenickel.

Many of these same challenges occur when a battery is operated at hightemperature. That is, conventional electrolytes can decompose byoxidation or may be degraded by reduction at high temperature analogousto the way these mechanisms affect the electrolytes at high voltage.Other parasitic reactions can also occur at elevated temperature.

As disclosed herein, these challenges and others are addressed in highenergy lithium ion secondary batteries including cathode activematerials that are capable of operation at high voltage and/or hightemperature.

BRIEF SUMMARY OF THE INVENTION

Certain embodiments relate to a battery including an anode, a cathode,and an electrolyte formulation including a lithium salt, a non-aqueoussolvent, and an ester-containing polymer additives. The cathode materialcan be an NMC material.

Preferred ester-containing polymer additives include poly(vinyl acetate)(“PVA”) and the polymer additive poly(1,4-butylene adipate) (“PBA”).

In some embodiments, polymers having an alkyl group or substituted alkylgroup at the methyl position of poly(vinyl acetate) can be suitablepolymer additives for electrolyte solutions are preferred.

Certain embodiments include methods making, using, and conditioning suchbatteries for use.

Other aspects and embodiments of the invention are also contemplated.The foregoing summary and the following detailed description are notmeant to restrict the invention to any particular embodiment but aremerely meant to describe some embodiments of the invention.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a lithium ion battery implemented in accordance withan embodiment of the invention.

FIG. 2 illustrates the operation of a lithium ion battery and agraphical representation of an illustrative non-limiting mechanism ofaction of an electrolyte including an additive compound, according to anembodiment of the invention.

FIG. 3 illustrates the high temperature cycle life testing of anelectrolyte formulation according to certain embodiments of theinvention.

FIG. 4 illustrates the high temperature cycle life testing of anelectrolyte formulation according to certain embodiments of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions apply to some of the aspects described withrespect to some embodiments of the invention. These definitions maylikewise be expanded upon herein. Each term is further explained andexemplified throughout the description, figures, and examples. Anyinterpretation of the terms in this description should take into accountthe full description, figures, and examples presented herein.

The singular terms “a,” “an,” and “the” include the plural unless thecontext clearly dictates otherwise. Thus, for example, reference to anobject can include multiple objects unless the context clearly dictatesotherwise.

The terms “substantially” and “substantial” refer to a considerabledegree or extent. When used in conjunction with an event orcircumstance, the terms can refer to instances in which the event orcircumstance occurs precisely as well as instances in which the event orcircumstance occurs to a close approximation, such as accounting fortypical tolerance levels or variability of the embodiments describedherein.

The term “about” refers to the range of values approximately near thegiven value in order to account for typical tolerance levels,measurement precision, or other variability of the embodiments describedherein.

The term “specific capacity” refers to the amount (e.g., total ormaximum amount) of electrons or lithium ions a material is able to hold(or discharge) per unit mass and can be expressed in units of mAh/g. Incertain aspects and embodiments, specific capacity can be measured in aconstant current discharge (or charge) analysis, which includesdischarge (or charge) at a defined rate over a defined voltage rangeagainst a defined counter electrode. For example, specific capacity canbe measured upon discharge at a rate of about 0.05 C (e.g., about 8.75mA/g) from 4.45 V to 3.0 V versus a Li/Li⁺ counter electrode. Otherdischarge rates and other voltage ranges also can be used, such as arate of about 0.1 C (e.g., about 17.5 mA/g), or about 0.5 C (e.g., about87.5 mA/g), or about 1.0 C (e.g., about 175 mA/g).

A rate “C” refers to either (depending on context) the discharge currentas a fraction or multiple relative to a “1 C” current value under whicha battery (in a substantially fully charged state) would substantiallyfully discharge in one hour, or the charge current as a fraction ormultiple relative to a “1 C” current value under which the battery (in asubstantially fully discharged state) would substantially fully chargein one hour.

The term “coulombic efficiency” is sometimes abbreviated herein as CEand refers the efficiency with which charge is transferred in a givencycle.

The term “rated charge voltage” refers to an upper end of a voltagerange during operation of a battery, such as a maximum voltage duringcharging, discharging, and/or cycling of the battery. In some aspectsand some embodiments, a rated charge voltage refers to a maximum voltageupon charging a battery from a substantially fully discharged statethrough its (maximum) specific capacity at an initial cycle, such as the1st cycle, the 2nd cycle, or the 3rd cycle. In some aspects and someembodiments, a rated charge voltage refers to a maximum voltage duringoperation of a battery to substantially maintain one or more of itsperformance characteristics, such as one or more of coulombicefficiency, retention of specific capacity, retention of energy density,and rate capability.

The term “rated cut-off voltage” refers to a lower end of a voltagerange during operation of a battery, such as a minimum voltage duringcharging, discharging, and/or cycling of the battery. In some aspectsand some embodiments, a rated cut-off voltage refers to a minimumvoltage upon discharging a battery from a substantially fully chargedstate through its (maximum) specific capacity at an initial cycle, suchas the 1st cycle, the 2nd cycle, or the 3rd cycle, and, in such aspectsand embodiments, a rated cut-off voltage also can be referred to as arated discharge voltage. In some aspects and some embodiments, a ratedcut-off voltage refers to a minimum voltage during operation of abattery to substantially maintain one or more of its performancecharacteristics, such as one or more of coulombic efficiency, retentionof specific capacity, retention of energy density, and rate capability.

The “maximum voltage” refers to the voltage at which both the anode andthe cathode are fully charged. In an electrochemical cell, eachelectrode may have a given specific capacity and one of the electrodeswill be the limiting electrode such that one electrode will be fullycharged and the other will be as fully charged as it can be for thatspecific pairing of electrodes. The process of matching the specificcapacities of the electrodes to achieve the desired capacity of theelectrochemical cell is “capacity matching.”

The term “NMC” refers generally to cathode materials containingLiNi_(x)Mn_(y)Co_(z)O_(w) and includes, but is not limited to, cathodematerials containing LiNi_(0.33)Mn_(0.33)Co_(0.33)O₂,LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂, and LiNi_(0.4)Mn_(0.4)Co_(0.2)O₂.

The term “polymer” refers generally to a molecule whose structure iscomposed of multiple repeating units. The structure can be linear orbranched. In the chemical formulas depicted herein, the subscripts “m”and “n” refer to the number of repeating units and are positiveintegers. The term polymer includes homopolymers and copolymers as thoseterms are used herein.

The term “homopolymer” refers to a polymer that is made bypolymerization of a single monomer.

The term “copolymer” refers generally to a molecule whose structure iscomposed of at least two different repeating units. The structure can bealternating, periodic, statistical, random, block, linear, branched,combinations thereof, or other structure. In certain embodimentsdisclosed herein, the copolymer is preferably a block copolymer. Incertain embodiments disclosed herein, the copolymer is preferably arandom copolymer. In certain embodiments disclosed herein, the copolymeris preferably a branched copolymer.

As used herein, the term “moiety” refers to a distinct, structurallyidentifiable, structurally isolated, or structurally named portion of amolecule.

To the extent certain battery characteristics can vary with temperature,such characteristics are specified at room temperature (about 30 degreesC.), unless the context clearly dictates otherwise.

Ranges presented herein are assumed to be inclusive of their endpointsunless context or mathematical symbols indicated otherwise. Thus, forexample, the range 1 to 3 includes the values 1 and 3 as well asintermediate values.

FIG. 1 illustrates a lithium ion battery 100 implemented in accordancewith an embodiment of the invention. The battery 100 includes an anode102, a cathode 106, and a separator 108 that is disposed between theanode 102 and the cathode 106. In the illustrated embodiment, thebattery 100 also includes a high voltage electrolyte 104, which isdisposed within and between the anode 102 and the cathode 106 andremains stable during high voltage battery cycling.

The operation of the battery 100 is based upon reversible intercalationand de-intercalation of lithium ions into and from host materials of theanode 102 and the cathode 106. Other implementations of the battery 100are contemplated, such as those based on conversion chemistry. Referringto FIG. 1, the voltage of the battery 100 is based on redox potentialsof the anode 102 and the cathode 106, where lithium ions areaccommodated or released at a lower potential in the former and a higherpotential in the latter. To allow both a higher energy density and ahigher voltage platform to deliver that energy, the cathode 106 caninclude an active cathode material for high voltage operations at orabove 4.3V.

Examples of suitable cathode materials include phosphates,fluorophosphates, fluorosulfates, fluorosilicates, spinels, lithium-richlayered oxides, and composite layered oxides. Further examples ofsuitable cathode materials include: spinel structure lithium metaloxides, layered structure lithium metal oxides, lithium-rich layeredstructured lithium metal oxides, lithium metal silicates, lithium metalphosphates, metal fluorides, metal oxides, sulfur, and metal sulfides.Examples of suitable anode materials include conventional anodematerials used in lithium ion batteries, such as lithium, graphite(“Li_(x)C₆”), and other carbon, silicon, or oxide-based anode materials.

FIG. 2 illustrates operation of a lithium ion battery and anillustrative, non-limiting mechanism of action of an improvedelectrolyte, according to an embodiment of the invention. Without beingbound by a particular theory not recited in the claims, the inclusion ofone or more stabilizing additive compounds in an electrolyte solutioncan, upon operation of the battery (e.g., during conditioning thereof),passivate a cathode material, thereby reducing or preventing reactionsbetween bulk electrolyte components and the cathode material that candegrade battery performance.

Referring to FIG. 2, an electrolyte 202 includes a base electrolyte,and, during initial battery cycling, components within the baseelectrolyte can assist in the in-situ formation of a protective film (inthe form of a solid electrolyte interface (“SEI”) 206) on or next to ananode 204. The anode SEI 206 can inhibit reductive decomposition of theelectrolyte 202. Preferably, and without being bound by theory notrecited in the claims, for operation at voltages at or above 4.2 V, theelectrolyte 202 can also include additives that can assist in thein-situ formation of a protective film (in the form of a SEI 208 oranother derivative) on or next to a cathode 200. The cathode SEI 208 caninhibit oxidative decomposition of the high voltage electrolyte 202 thatcan otherwise occur during high voltage operations. As such, the cathodeSEI 208 can inhibit oxidative reactions in a counterpart manner to theinhibition of reductive reactions by the anode SEI 206. In theillustrated embodiment, the cathode SEI 208 can have a thickness in thesub-micron range, and can include one or more chemical elementscorresponding to, or derived from, those present in one or moreadditives, such as silicon or other heteroatom included in one or moreadditives. Advantageously, one or more additives can preferentiallypassivate the cathode 200 and can selectively contribute towards filmformation on the cathode 200, rather than the anode 204. Suchpreferential or selective film formation on the cathode 200 can impartstability against oxidative decomposition, with little or no additionalfilm formation on the anode 204 (beyond the anode SEI 206) that canotherwise degrade battery performance through resistive losses. Moregenerally, one or more additives can decompose below a redox potentialof the cathode material and above a redox potential of SEI formation onthe anode 204.

Without being bound by a particular theory not recited in the claims,the formation of the cathode SEI 208 can occur through one or more ofthe following mechanisms: (1) the additive compound(s) can decompose toform the cathode SEI 208, which inhibits further oxidative decompositionof electrolyte components; (2) the additive compound(s) or itsdecomposed product(s) form or improve the quality of a passivation filmon the cathode or anode; (3) the additive compounds can form anintermediate product, such as a complex with LiPF₆ or a cathodematerial, which intermediate product then decomposes to form the cathodeSEI 208 that inhibits further oxidative decomposition of electrolytecomponents; (4) the additive compounds can form an intermediate product,such as a complex with LiPF₆, which then decomposes during initialcharging. The resulting decomposition product can then further decomposeduring initial charging to form the cathode SEI 208, which inhibitsfurther oxidative decomposition of electrolyte components; (5) theadditive compounds can stabilize the cathode material by preventingmetal ion dissolution.

Other mechanisms of action of the electrolyte 202 are contemplated,according to an embodiment of the invention. For example, and in placeof, or in combination with, forming or improving the quality of thecathode SEI 208, one or more additives or a derivative thereof can formor improve the quality of the anode SEI 206, such as to reduce theresistance for lithium ion diffusion through the anode SEI 206. Asanother example, one or more additives or a derivative thereof canimprove the stability of the electrolyte 202 by chemically reacting orforming a complex with other electrolyte components. As a furtherexample, one or more additives or a derivative thereof can scavengedecomposition products of other electrolyte components or dissolvedelectrode materials in the electrolyte 202 by chemical reaction orcomplex formation. Any one or more of the cathode SEI 208, the anode SEI206, and the other decomposition products or complexes can be viewed asderivatives, which can include one or more chemical elementscorresponding to, or derived from, those present in one or moreadditives, such as a heteroatom included in the additives.

Certain embodiments are related to a class of polymeric additives fornon-aqueous electrolytes. Such embodiments include several electrolyteadditives that improve the oxidative stability of the electrolyte andthe cycle life and coulombic efficiency of electrochemical cellscontaining these additives.

A high voltage electrolyte according to some embodiments of theinvention can be formed with reference to the formula:

base electrolyte+additive→high voltage electrolyte   (1)

A high temperature electrolyte according to some embodiments of theinvention can be formed with reference to the formula:

base electrolyte+additive→high temperature electrolyte   (2)

In formulas (1) and (2), the base electrolyte can include one or moresolvents and one or more salts, such as lithium-containing salts in thecase of lithium ion batteries. Examples of suitable solvents includenonaqueous electrolyte solvents for use in lithium ion batteries,including carbonates, such as ethylene carbonate, dimethyl carbonate,ethyl methyl carbonate, propylene carbonate, methyl propyl carbonate,and diethyl carbonate; sulfones; silanes; nitriles; esters; ethers; andcombinations thereof. The base electrolyte can also include additionalsmall molecule additives.

Referring to formulas (1) and (2), an amount of a particular additivecan be expressed in terms of a weight percent of the additive relativeto a total weight of the electrolyte solution (or wt. %). For example,an amount of an additive can be in the range of about 0.01 wt. % toabout 30 wt. %, such as from about 0.05 wt. % to about 30 wt. %, fromabout 0.01 wt. % to about 20 wt. %, from about 0.2 wt. % to about 15 wt.%, from about 0.2 wt. % to about 10 wt. %, from about 0.2 wt. % to about5 wt. %, or from about 0.2 wt. % to about 1 wt. %, and, in the case of acombination of multiple additive, a total amount of the additive can bein the range of about 0.01 wt. % to about 30 wt. %, such as from about0.05 wt. % to about 30 wt.%, from about 0.01 wt. % to about 20 wt. %,from about 0.2 wt. to about 15 wt. %, from about 0.2 wt. % to about 10wt. %, from about 0.2 wt. % to about 5 wt. %, or from about 0.2 wt. % toabout 1 wt. %. An amount of an additive also can be expressed in termsof a ratio of the number of moles of the additive per unit surface areaof either, or both, electrode materials. For example, an amount of acompound can be in the range of about 10⁻⁷ mol/m² to about 10⁻²mol/m²,such as from about 10⁻⁷ mol/m²to about 10⁻⁵ mol/m², from about 10⁻⁵mol/m² to about 10⁻³ mol/m², from about 10⁻⁶ mol/m² to about 10−4mol/m², or from about 10⁻⁴ mol/m² to about 10⁻² mol/m². As furtherdescribed below, a additive can be consumed or can react, decompose, orundergo other modifications during initial battery cycling. As such, anamount of a compound can refer to an initial amount of the compound usedduring the formation of the electrolyte solutions according to formulas(1) or (2), or can refer to an initial amount of the additive within theelectrolyte solution prior to battery cycling (or prior to anysignificant amount of battery cycling).

Resulting performance characteristics of a battery can depend upon theidentity of a particular additive used to form the high voltageelectrolyte according to formulas (1) or (2), an amount of the compoundused, and, in the case of a combination of multiple compounds, arelative amount of each compound within the combination. Accordingly,the resulting performance characteristics can be fine-tuned or optimizedby proper selection of the compounds and adjusting amounts of thecompounds in formulas (1) or (2).

The formation according to formulas (1) or (2) can be carried out usinga variety of techniques, such as by mixing the base electrolyte and theadditives, dispersing the additives within the base electrolyte,dissolving the additives within the base electrolyte, or otherwiseplacing these components in contact with one another. The additives canbe provided in a liquid form, a powdered form (or another solid form),or a combination thereof. The additives can be incorporated in theelectrolyte solutions of formulas (1) or (2) prior to, during, orsubsequent to battery assembly.

The electrolyte solutions described herein can be used for a variety ofbatteries containing a high voltage cathode or a low voltage cathode,and in batteries operated at high temperatures. For example, theelectrolyte solutions can be substituted in place of, or used inconjunction with, conventional electrolytes for lithium ion batteriesfor operations at or above 4.2 V. In particular, these additives areuseful for lithium ion batteries containing NMC cathode materials.

Batteries including the electrolyte solutions can be conditioned bycycling prior to commercial sale or use in commerce. Such conditioningcan include, for example, providing a battery, and cycling such batterythrough at least 1, at least 2, at least 3, at least 4, or at least 5cycles, each cycle including charging the battery and discharging thebattery at a rate of 0.05 C (e.g., a current of 8.75 mA/g) between 4.45Vand 3.0V (or another voltage range) versus a reference counterelectrode, such as a graphite anode. Charging and discharging can becarried out at a higher or lower rate, such as at a rate of 0.1 C (e.g.,a current of 17.5 mA/g), at a rate of 0.5 C (e.g., a current of 87.5mA/g), or at a rate of 1 C (e.g., a current of 175 mA/g). Typically abattery is conditioned with 1 cycle by charging at 0.05 C rate to 4.45Vfollowed by applying constant voltage until the current reaches 0.02 C,and then discharging at 0.05 C rate to 3V.

The polymer additives according to embodiments herein are moleculesformed from numerous repeated monomer units, as is conventionallyunderstood in the art. Such polymer additives contain certain functionalgroups along the backbone of the polymer chain.

The polymers disclosed herein are generally referred to by the names ofthe monomer molecules that are used to synthesize the polymer. While thepolymers referred to herein generally are named by the monomers used toform them, it is possible that some of the polymers could be referred toby alternate names. The disclosure is intended to encompass suchvariations in chemical nomenclature without departing from the scope andspirit of the invention.

The number of repeat units in the polymers disclosed herein is greaterthan 1; and is in some cases greater than 100; in some cases greaterthan 250; in some cases greater than 500; in some cases greater than1,000; in some cases greater than 5,000; in some cases greater than10,000; in some cases greater than 50,000; in some cases greater than100,000; in some cases greater than 500,000; and in some cases greaterthan 1,000,000.

The number of repeat units in the polymers disclosed herein is greaterthan 1; and is in some cases less than 100; in some cases less than 250;in some cases less than 500; in some cases less than 1,000; in somecases less than 5,000; in some cases less than 10,000; in some casesless than 50,000; in some cases less than 100,000; in some cases lessthan 500,000; and in some cases less than 1,000,000.

In certain embodiments disclosed herein, ester-containing polymers areused as additives to a conventional electrolyte solution for a lithiumion battery. In some embodiments, the polymers described herein aresynthesized from polymeric reactions using a reactive ester polymerprecursor. These polymer additives can form more mechanically andchemically stable SEI films compared to small molecule or short-chainoligomers, resulting in improved cycle life at high temperatures andhigh voltages. The results presented herein demonstrate that the polymeradditives having ester functionality can significantly improve batteryperformance as compared to a control electrolyte without the polymeradditive.

Certain ester-containing polymers are preferred as electrolyteadditives. For example, poly(vinyl acetate), which can be represented byFormula (a), is a preferred electrolyte additive in certain embodimentsof the invention:

More broadly, polymers having an alkyl group or substituted alkyl groupat the methyl position of poly(vinyl acetate) can be suitable polymeradditives for electrolyte solutions. For example, Formula (b)illustrates a chemical structure where Ri can be an alkyl group. As usedherein alkyl groups include lower alkyls (an alkyl that includes from 2to 20 carbon atoms, such as from 2 to 10 carbon atoms), upper alkyls (analkyl that includes more than 20 carbon atoms, such as from 21 to 100carbon atoms), cycloalkyls (an alkyl that includes one or more ringstructures), heteroalkyls (an alkyl that has one or more of its carbonatoms replaced by one or more heteroatoms, such as N, Si, S, O, F, andP), and branched forms of all such alkyls. Alkyls can be substitutedsuch that one or more of its hydrogen atoms is replaced by one or moresubstituent groups, such as halo groups. An alkyl can have a combinationof characteristics. Examples of alkyl groups include methyl, ethyl,n-propyl, isopropyl, cyclopropyl, butyl, isobutyl, t-butyl, cyclobutyl,and hetero, or substituted forms thereof.

Formula (b) is:

Another example of a preferred electrolyte additive in certainembodiments of the invention is the polymer poly(1,4-butylene adipate),which is represented by Formula (c):

Simple substitutions along the backbone of the polymer depicted inFormula (c) are included in this disclosure.

Certain properties are preferred in polymer additives for use inelectrochemical cells. For example, the additives preferably are: (i)either chemically resistant to oxidation and/or reduction under the cellconditions or, if not chemically resistant to oxidation and/orreduction, then the additives should decompose to intermediates orproducts that form a stable SEI film on the anode, cathode, or both; and(ii) sufficiently soluble in electrolyte solution at room temperatureand to make the electrolyte solution viscosity during battery operationnot worse than without the additive.

In addressing the challenges of high energy cathode materials, theadditives according to embodiments disclosed herein have a number ofbenefits, including: (i) unique functional groups pre-arranged in thebackbone, which allows the polymer additives to strongly and evenlyadsorb on to the surface of the electrodes before decomposition andpotentially improving the quality and stability of the resulting SEI;and (ii) mechanical and chemical stability as compared to organicoligomers and short-chain polymers formed from conventional solvents andadditives due to the pre-formed polymer backbone.

Further, the high molecular weight SEI species resulting from reactionsof the ring-opening decomposition of the succinic anhydride moiety withlithium salt species can: (i) be homogenously dispersed throughout theSEI to form a more uniform film; (ii) provide a more mechanically andchemically stable SEI on both cathode and anode surface; (iii) be usedto chelate cathode transition metal ions dissolved in the electrolyte,which prevents anode SEI breakdown leading to capacity fade; and (iv)function as scavenger or acidic reactive species and/or protonicreactive species, which decreases chain reactions of solvent and SEIdecomposition caused by those reactive species.

In certain embodiments of the invention, the additive is present at anamount that is significantly lower than the amount of electrolyte saltpresent in the electrolyte formulation of the electrochemical cell. Theamount of additive can be expressed as a weight percent (wt %) of thetotal weight of the electrolyte formulation. In certain embodiments ofthe invention, the concentration of additive in the electronicformulation is less than or equal to the concentration at which theadditive would be at the saturation point in the electrolyte solvent. Incertain embodiments of the invention, the concentration of additive inthe electronic formulation is less than or equal to about 10 weightpercent, more preferably less than or equal to about 9 weight percent,more preferably less than or equal to about 8 weight percent, morepreferably less than or equal to about 7 weight percent, more preferablyless than or equal to about 6 weight percent, more preferably less thanor equal to about 5 weight percent, more preferably less than or equalto about 4 weight percent, more preferably less than or equal to about 3weight percent, and still more preferably less than or equal to about 2weight percent.

In certain embodiments of the invention, the concentration of eachadditive in the electronic formulation is equal to about 10.0 wt %, 9.9wt %, 9.8 wt %, 9.7 wt %, 9.6 wt %, 9.5 wt %, 9.4 wt %, 9.3 wt %, 9.2 wt%, 9.1 wt %, 9.0 wt %, 8.9 wt %, 8.8 wt %, 8.7 wt %, 8.6 wt %, 8.5 wt %,8.4 wt %, 8.3 wt %, 8.2 wt %, 8.1 wt %, 8.0 wt %, 7.9 wt %, 7.8 wt %,7.7 wt %, 7.6 wt %, 7.5 wt %, 7.4 wt %, 7.3 wt %, 7.2 wt %, 7.1 wt %,7.0 wt %, 6.9 wt %, 6.8 wt %, 6.7 wt %, 6.6 wt %, 6.5 wt %, 6.4 wt %,6.3 wt %, 6.2 wt %, 6.1 wt %, 6.0 wt %, 5.9 wt %, 5.8 wt %, 5.7 wt %,5.6 wt %, 5.5 wt %, 5.4 wt %, 5.3 wt %, 5.2 wt %, 5.1 wt %, 5.0 wt %,4.9 wt %, 4.8 wt %, 4.7 wt %, 4.6 wt %, 4.5 wt %, 4.4 wt %, 4.3 wt %,4.2 wt %, 4.1 wt %, 4.0 wt %, 3.9 wt %, 3.8 wt %, 3.7 wt %, 3.6 wt %,3.5 wt %, 3.4 wt %, 3.3 wt %, 3.2 wt %, 3.1 wt %, 3.0 wt %, 2.9 wt %,2.8 wt %, 2.7 wt %, 2.6 wt %, 2.5 wt %, 2.4 wt %, 2.3 wt %, 2.2 wt %, or2.1 wt %, 2.0 wt %, 1.9 wt %, 1.8 wt %, 1.7 wt %, 1.6 wt %, 1.5 wt %,1.4 wt %, 1.3 wt %, 1.2 wt %, 1.1 wt %, 1.0 wt %, 0.9 wt %, 0.8 wt %,0.7 wt %, 0.6 wt %, 0.5 wt %, 0.4 wt %, 0.3 wt %, 0.2 wt %, or 0.1 wt %.In certain embodiments of the invention, the concentration of additivein the electrolyte formulation is in the range of about 2.0 wt % toabout 0.5 wt %.

The following examples and methods describe specific aspects of someembodiments of the invention to illustrate and provide a description forthose of ordinary skill in the art. The examples and methods should notbe construed as limiting the invention, as the examples and methodsmerely provide specific methodology useful in understanding andpracticing some embodiments of the invention.

Methods

Battery Cell Assembly. Battery cells were formed in a high purity Argonfilled glove box (M-Braun, O₂ and humidity content <0.1 ppm). In thecase of the cathode, a commercial LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ (referredto herein as NMC 532) or LiNi_(0.4)Mn_(0.4)Co_(0.2)O₂ (referred toherein as NMC 442) cathode material was mixed with dry poly(vinylidenefluoride), carbon black powder, and liquid 1-methyl-2-pyrrolidinone toform a slurry. The resulting slurry was deposited on an aluminum currentcollector and dried to form a composite cathode film. In the case of theanode, a graphitic carbon was mixed with dry poly(vinylidene fluoride),carbon black powder, and liquid 1-methyl-2-pyrrolidinone to form aslurry. The resulting slurry was deposited on a copper current collectorand dried to form a composite anode film. Each battery cell included thecomposite cathode film, a polypropylene separator, and composite anodefilm. A conventional electrolyte formed from 1 M of LiPF6 in ethylenecarbonate and ethyl methyl carbonate (EC:EMC=1:2) by volume was mixedwith the desired weight percentage of an embodiment of the inventiveadditive and added to the battery cell. The battery cell was sealed andinitially cycled at ambient temperature using 0.1 C charge to uppercutoff voltage (up to 4.4V) followed by constant voltage hold until thecurrent dropped to 0.05 C and then discharged to 2.8V using 0.1 Cconstant current. The cycle was repeated one more time prior to hightemperature cycling.

High Temperature Testing. Test batteries were cycled up to 4.4V in anenvironment at a temperature of about 40 degrees Celsius using 0.5 Ccharge followed by constant voltage hold for 1 hour and then dischargedto 2.8V using 0.5 C constant current. Table 1 shows certain data for thecycle life testing of some embodiments of the additives disclosed hereinas compared to control and FIGS. 3 and 4 show the full cycle lifetesting.

Specifically, data was collected on the polymer additive poly(vinylacetate) (“PVA”) with a molecular weight around 100,000 g/mol and thepolymer additive poly(1,4-butylene adipate) (“PBA”) with a molecularweight around 12,000 g/mol. These molecular weight values are weightaverage molecular weight determined by gel permeation chromatography (orsize exclusion chromatography). For PVA, a preferred range for weightaverage molecular weight is from about 50,000 g/mol to about 500,000g/mol. For PBA, a preferred range for weight average molecular weight isfrom about 1,000 g/mol to about 40,000 g/mol.

TABLE 1 Summary of additive performance compared to the controlelectrolyte 200th Cycle 1st Cycle Capacity Capacity, 1st CycleRetention, Polymer 30° C. CE 40° C. Additives Cell Chemistry (mAh/g) (%)(%,) None 4.3 V NMC 532/ 180 88 82.6 (Control) Graphite 0.5% PVA 4.3 VNMC 532/ 183 89 88.9 Graphite None 4.4 V NMC 442/ 193 87 33.8 (Control)Graphite 2.0% PBA 4.4 V NMC 442/ 194 86 67.9 Graphite

The data in Table 1 demonstrate that batteries using ester-containingpolymer additives in the electrolyte solution have significantlyimproved high temperature capacity retention as compared to batterieswithout the ester-containing polymer additives in the electrolytesolution. Each of the batteries using ester-containing polymer additivesin the electrolyte solution improved the capacity retention at the 200thcycle at high temperature as compared to the control battery.

FIG. 3 illustrates the high temperature cycle life testing of anelectrolyte formulation including poly(vinyl acetate) as an additive.The battery included a NMC532 composite cathode and a graphite compositeanode. The battery was cycled from 2.8V to 4.3V in an environment at atemperature of about 40 degrees Celsius. FIG. 3 demonstrates a reductionin capacity retention beginning around cycle 40 for the control batterywhile the test battery with the polymer additive in the electrolytesolution performs better than the control battery throughout the hightemperature testing.

FIG. 4 illustrates the high temperature cycle life testing of anelectrolyte formulation including poly(1,4-butylene adipate) as anadditive. The battery included a NMC422 composite cathode and a graphitecomposite anode. The battery was cycled from 2.8V to 4.4V in anenvironment at a temperature of about 40 degrees Celsius. FIG. 3demonstrates a dramatic reduction in capacity retention around cycle 60for the control battery while the test battery with the polymer additivein the electrolyte solution performs significantly better than thecontrol battery throughout the high temperature testing.

The data presented herein confirm that certain polymer additives canprovide significant improvements to the high temperature capacityretention when added to electrolyte formulations. Specifically,batteries including an NMC composite cathode and graphite compositeanode showed improvements as compared to a comparable battery withoutthe polymer additives in the electrolyte formulation. Notably, theinitial performance of the tested batteries was similar to the controlbatteries, which indicates that the additives do not have a negativeimpact on the cell capacity despite providing improved capacityretention.

While the invention has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the invention asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, or process to the objective, spirit and scope of the invention.All such modifications are intended to be within the scope of the claimsappended hereto. In particular, while the methods disclosed herein havebeen described with reference to particular operations performed in aparticular order, it will be understood that these operations may becombined, sub-divided, or re-ordered to form an equivalent methodwithout departing from the teachings of the invention. Accordingly,unless specifically indicated herein, the order and grouping of theoperations are not limitations of the invention.

What is claimed is:
 1. An electrolyte formulation, comprising: a lithiumsalt, a non-aqueous solvent, and a polymer additive represented byFormula (c):

where n is an integer greater than
 1. 2. The electrolyte formulation ofclaim 1, wherein the weight average molecular of the polymer additive isless than or equal to 12,000 g/mol.
 3. The electrolyte formulation ofclaim 1, wherein the polymer additive is present at a concentration ofat least 0.1 weight percent of the total weight of the electrolyteformulation.
 4. The electrolyte formulation of claim 1, wherein thepolymer additive is present at a concentration of at least 0.5 weightpercent of the total weight of the electrolyte formulation.
 5. Theelectrolyte formulation of claim 1, wherein the polymer additive is ahomopolymer.